The present invention relates to an adhesive prepared from polymeric adhesion promoters such as polyamidoamine, and latexes. The adhesive is useful between various substrates such as between plastic films.
The present invention also relates to polymeric latex based adhesives which contain a polymeric adhesion promoter and epoxy emulsion to be used to laminate various substrates, such as plastic films.
Most plastic film is adhered to other plastic films and paperboard by use of solvent based adhesives and not by use of aqueous based systems such as latex systems.
Multilayer films are presently used in various types of packaging. Food items in particular are packaged in multilayer film packages. Individual films having different properties contribute those properties to the multilayer structure. For example, one film may have particularly good oxygen barrier properties while another may have excellent tensile strength.
The multilayer structure can be manufactured by adhesive lamination, coextrusion or extrusion lamination. The preferred multilayer structure of the present invention is the use of the adhesive in lamination. Typically films are laminated to each other by coating the primary film or web with an adhesive. The secondary web is then laminated to the primary web after the adhesive is dried.
Multilayer films are used in the food packaging industry for packaging various foodstuffs such as snack food, and processed meat and cheese. These meat and cheese packages are often exposed to high humidity. Therefore a retention of bond strength in high humidity is a desirable property in the laminated structure.
Solvent-born adhesives which impart strong bond retention between substrates in high humidity are normally used in adhering most plastic film to plastic films and other substrates. The good bond strength occurs with timer typically with a crosslinking reaction in the adhesive. Since the crosslinking does not occur instantaneously, the green strength (initial strength) of the bond could be poor and therefore undesirable. Such systems also result in difficult handling of the adhesive because the pot-life of such systems is relatively short.
The additional disadvantage to using solvent-based adhesives is solvent emission. The increased need for water based adhesives in an attempt to eliminate solvent emissions, necessitates an efficient and economical method of latex application to bond plastic substrates such as plastic film to plastic films, to paperboard or to other substrates without sacrificing the strength of the bond.
Other known adhesives in addition to the solvent based adhesives are latex based laminating adhesives which normally contain a wetting agent and a crosslinking agent. These systems typically have similar disadvantages to the solvent based systems, i.e. a short pot-life and insufficient green strength. The latex based adhesives also contain hydrophilic additives which impart wetting properties and latex stability. However, humidity will also weaken such latex based adhesive bonds.
In view of these disadvantages of present adhesive systems, it is desirable to provide an efficient latex aqueous adhesive to bond plastic substrates, such as plastic film to plastic films, to paper board and other substrates without sacrificing the initial strength of the bond or the strength of the bond after an elapsed period of time in both dry and humid environments and without sacrificing the pot-life of the adhesive.